Transport refrigeration systems are used for transporting perishable goods, such as refrigerated and frozen food products. Transport refrigeration systems include refrigerated containers, trucks, and railroad cars. Some products require more accurate temperature control of the refrigerated compartment than others to preserve product freshness. For example, some frozen foods may need only to be kept below a certain freezing temperature, with less sensitivity to a specific set point temperature. Other goods, such as some perishable produce such as fruits or vegetables might require a tighter temperature regulation to preserve optimal product freshness.
Transport refrigeration systems can be divided into two or more compartments by inserting an internal wall. The individual spaces can be kept at different temperatures. For example, one compartment can be a freezer compartment and the other compartment can be refrigerated. Typically such dual compartment shipping container systems use one refrigeration compressor and two evaporators, one for each compartment. While the primary compartment might have a proportional refrigerant pressure control, the existing method of secondary compartment temperature control is to cycle on and off the liquid refrigerant line to the secondary compartment evaporator. This method of cycling the secondary evaporator liquid refrigerant line on and off to control the temperature in the secondary compartment cannot achieve the temperature regulation tolerance that is needed in many applications. Therefore what is needed is a method and apparatus to improve the temperature regulation in a secondary refrigerated compartment.
Another problem involving multiple compartment transport refrigeration systems is how to apportion the available cooling capacity at startup and/or under high load conditions, such as when the ambient temperature is very high. What is needed is a control algorithm to apportion available cooling capacity by a priority system between a primary compartment and a secondary compartment.
Yet another problem is to limit the electrical power consumption of a multiple compartment transport refrigeration system at startup and/or under high load conditions, such as when the ambient temperature is very high. For example, ocean going container ships may have power limits and/or circuit breakers that limit the ampacity of the electrical power supply line to each refrigerated transport container. A typical current limit is 15 to 23 amperes, with circuit interruption protection typically set to 30 amperes (maximum). Therefore, what is also needed is a method of control for a multi-compartment transport refrigeration system that can limit the electrical load to a preset value while apportioning the resulting cooling capacity between the compartments.